Tcl

Tcl(n)                       Tcl Built-In Commands                      Tcl(n)



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NAME
       Tcl - Summary of Tcl language syntax.
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DESCRIPTION
       The following rules define the syntax and semantics of the Tcl
       language:

       [1]    A Tcl script is a string containing one or more commands.  Semi-
              colons and newlines are command separators unless quoted as
              described below.  Close brackets are command terminators during
              command substitution (see below) unless quoted.

       [2]    A command is evaluated in two steps.  First, the Tcl interpreter
              breaks the command into words and performs substitutions as
              described below.  These substitutions are performed in the same
              way for all commands.  The first word is used to locate a
              command procedure to carry out the command, then all of the
              words of the command are passed to the command procedure.  The
              command procedure is free to interpret each of its words in any
              way it likes, such as an integer, variable name, list, or Tcl
              script.  Different commands interpret their words differently.

       [3]    Words of a command are separated by white space (except for
              newlines, which are command separators).

       [4]    If the first character of a word is double-quote (``"'') then
              the word is terminated by the next double-quote character.  If
              semi-colons, close brackets, or white space characters
              (including newlines) appear between the quotes then they are
              treated as ordinary characters and included in the word.
              Command substitution, variable substitution, and backslash
              substitution are performed on the characters between the quotes
              as described below.  The double-quotes are not retained as part
              of the word.

       [5]    If the first character of a word is an open brace (``{'') then
              the word is terminated by the matching close brace (``}'').
              Braces nest within the word: for each additional open brace
              there must be an additional close brace (however, if an open
              brace or close brace within the word is quoted with a backslash
              then it is not counted in locating the matching close brace).
              No substitutions are performed on the characters between the
              braces except for backslash-newline substitutions described
              below, nor do semi-colons, newlines, close brackets, or white
              space receive any special interpretation.  The word will consist
              of exactly the characters between the outer braces, not
              including the braces themselves.

       [6]    If a word contains an open bracket (``['') then Tcl performs
              command substitution.  To do this it invokes the Tcl interpreter
              recursively to process the characters following the open bracket
              as a Tcl script.  The script may contain any number of commands
              and must be terminated by a close bracket (``]'').  The result
              of the script (i.e. the result of its last command) is
              substituted into the word in place of the brackets and all of
              the characters between them.  There may be any number of command
              substitutions in a single word.  Command substitution is not
              performed on words enclosed in braces.

       [7]    If a word contains a dollar-sign (``$'') then Tcl performs
              variable substitution:  the dollar-sign and the following
              characters are replaced in the word by the value of a variable.
              Variable substitution may take any of the following forms:

              $name          Name is the name of a scalar variable;  the name
                             is terminated by any character that isn't a
                             letter, digit, or underscore.

              $name(index)   Name gives the name of an array variable and
                             index gives the name of an element within that
                             array.  Name must contain only letters, digits,
                             and underscores.  Command substitutions, variable
                             substitutions, and backslash substitutions are
                             performed on the characters of index.

              ${name}        Name is the name of a scalar variable.  It may
                             contain any characters whatsoever except for
                             close braces.

              There may be any number of variable substitutions in a single
              word.  Variable substitution is not performed on words enclosed
              in braces.

       [8]    If a backslash (``\'') appears within a word then backslash
              substitution occurs.  In all cases but those described below the
              backslash is dropped and the following character is treated as
              an ordinary character and included in the word.  This allows
              characters such as double quotes, close brackets, and dollar
              signs to be included in words without triggering special
              processing.  The following table lists the backslash sequences
              that are handled specially, along with the value that replaces
              each sequence.

              \a     Audible alert (bell) (0x7).

              \b     Backspace (0x8).

              \f     Form feed (0xc).

              \n     Newline (0xa).

              \r     Carriage-return (0xd).

              \t     Tab (0x9).

              \v     Vertical tab (0xb).

              \<newline>whiteSpace
                     A single space character replaces the backslash, newline,
                     and all spaces and tabs after the newline.  This
                     backslash sequence is unique in that it is replaced in a
                     separate pre-pass before the command is actually parsed.
                     This means that it will be replaced even when it occurs
                     between braces, and the resulting space will be treated
                     as a word separator if it isn't in braces or quotes.

              \\     Backslash (``\'').

              \ooo                                                             │
                     The digits ooo (one, two, or three of them) give an       │
                     eight-bit octal value for the Unicode character that will │
                     be inserted.  The upper bits of the Unicode character     │
                     will be 0.                                                │

              \xhh                                                             │
                     The hexadecimal digits hh give an eight-bit hexadecimal   │
                     value for the Unicode character that will be inserted.    │
                     Any number of hexadecimal digits may be present; however, │
                     all but the last two are ignored (the result is always a  │
                     one-byte quantity).  The upper bits of the Unicode        │
                     character will be 0.                                      │

              \uhhhh                                                           │
                     The hexadecimal digits hhhh (one, two, three, or four of  │
                     them) give a sixteen-bit hexadecimal value for the        │
                     Unicode character that will be inserted.

              Backslash substitution is not performed on words enclosed in
              braces, except for backslash-newline as described above.

       [9]    If a hash character (``#'') appears at a point where Tcl is
              expecting the first character of the first word of a command,
              then the hash character and the characters that follow it, up
              through the next newline, are treated as a comment and ignored.
              The comment character only has significance when it appears at
              the beginning of a command.

       [10]   Each character is processed exactly once by the Tcl interpreter
              as part of creating the words of a command.  For example, if
              variable substitution occurs then no further substitutions are
              performed on the value of the variable;  the value is inserted
              into the word verbatim.  If command substitution occurs then the
              nested command is processed entirely by the recursive call to
              the Tcl interpreter; no substitutions are performed before
              making the recursive call and no additional substitutions are
              performed on the result of the nested script.

       [11]   Substitutions do not affect the word boundaries of a command.
              For example, during variable substitution the entire value of
              the variable becomes part of a single word, even if the
              variable's value contains spaces.



Tcl                                   8.1                               Tcl(n)